The present invention relates to solid-oxide fuel cells and methods of making the same. More particularly, the present invention relates to concepts and processes for miniaturizing solid oxide fuel cells while maintaining substantial power output, thus increasing the power density.
A fuel cell is an electrochemical device in which electrical energy is generated by chemical reactions without altering the basic components of the fuel cellxe2x80x94the electrodes and the electrolyte. Fuel cells may be used to combine hydrogen or other reformed fuels with oxygen without combustion to produce direct current electric power. The process can be described as electrolysis in reverse, where the fuel cell converts chemical energy continuously into electrical energy without requiring an intermediate conversion to thermal energy.
Fuel cells have been pursued as a source of power for transportation because of their high energy efficiency, their potential for fuel flexibility, and their extremely low emissions. Fuel cells have potential for stationary and vehicular power generation applications; however, the commercial viability of fuel cells for power generation in these applications depends upon solving a number of manufacturing, cost, and durability problems. Despite decades of intensive cross-disciplinary research, the current cutting edge solid-oxide fuel cell (SOFC) technology is still deficient in establishing itself as a viable alternative to more traditional power generation. This deficiency is even more pronounced for mobile applications, where power density requirements are exceedingly high because of size and weight limitations for onboard fuel cells. The high efficiency, cleanliness, and inherent simplicity of SOFC technology captures the best properties of other approaches like the proton-exchange membrane (PEM) fuel cell, while providing the compatibility with existing fuel cell delivery infrastructure and the impurity tolerance required for mobile use. With current SOFC technology, the high operating temperature of fuel cell stacks requires complex and costly materials, and leads to exceedingly slow start-ups, which is incompatible with nominal vehicle operation. Even in the most advanced SOFC products available today, the power density is insufficient for most auxiliary-power unit applications, and only a fraction of what it needs to be for powertrain operations. Accordingly, there remains a need in this art for solid-oxide fuel cells, and processes for their fabrication that alleviate the above-mentioned problems.
A single fuel cell is comprised of several layers including an anode, electrolyte, cathode, and interconnect. One or more of these layers must contain a distribution system to deliver air and fuel to the electrodes. The volumetric power density is determined by the areal power density of a single fuel cell and the repeat distance for the stacking of said individual fuel cells. The areal power density is limited by resistive and polarization losses and possibly mass flow limitations. The losses typically increase with decreasing temperature. Under realistic operating conditions, a 0.5-1.0 W/cm2 areal power density (including gas manifolding) is reasonably close to the upper limit of current technology. The repeat distance for current technology is at best about 4 mm, partially due to the need for the fuel cells to be self-supporting, and also due to the use of macroscopic manufacturing techniques in the production of the fuel cell. These traditional fuel cells have volumetric power densities that at best have the potential to achieve 1-2 kW/L for hydrogen-fed fuel cells, i.e. disregarding reformers and packaging which will significantly lower this value, which should be viewed as an upper bound. The main idea of the present invention is a new strategy to improve volumetric power density by decreasing the repeat distance. This new fuel cell concept and the processes for its fabrication also alleviate many of the above-mentioned problems and limitations with durability and start-up.
The present invention provides a way of addressing the above mentioned problems by using a novel method for manufacturing miniaturized solid-oxide fuel cells so as to ensure sufficient power density. The present invention uses processes common to the manufacturing of microelectronics devices to shrink certain key dimensions of individual fuel cells and stacks of fuel cells in order to provide sufficient power density and durability.
Accordingly to one aspect of the invention, a miniaturized stacked fuel cell and process for its manufacture is provided wherein an interconnect is deposited on a sacrificial material, such as bulk YSZ, sapphire, or another refractory material which meets epitaxial and stability requirements. A first electrode layer is then deposited on the interconnect. An electrolyte material is deposited on the first electrode layer. Finally, a second electrode layer is deposited onto the electrolyte material. These steps are repeated, without the use of a sacrificial material, in order to fabricate a stacked fuel cell n times, where n is greater than 1. Preferably, n is as high as 1000. More Preferably n is greater than 1000.
A sacrificial material is used only once in the process, at the construction of the first individual fuel cell. The subsequent cells, which are stacked onto one another in the same manner as the first cell, are done so without an initial sacrificial support. Thus, the interconnect which begins the formation of the next fuel cell is deposited directly on the second electrode layer of the previous fuel cell. Manifolds for delivering reactants to anodes and cathodes in the fuel cell assembly can be placed in either the interconnect or in the electrode layers. If manifolding is done in interconnects, flow channels are created on both major sides of each interconnect. If manifolding is done in the electrodes, flow channels are created on one side only of each electrode, namely the one facing the interconnect.
The assembly of the miniaturized fuel cell is performed by various vapor deposition, lithography, and masking/etching techniques akin to processes used in the microelectronics industry to manufacture transistors and other components. The repeat distance of the stacked fuel cell is generally less than about 0.5 mm, a nearly 10-fold reduction compared to conventional fuel cells. Further reductions in repeat distance and concomitant increase in power density is possible if pressure drops in the manifolds can be kept at an acceptable level. An interlayer may be placed between either electrode and the electrolyte to improve areal power density. The interlayer may be about 0.001 to about 1 micron thick, and in one preferred embodiment consist of isovalently or aliovalently doped ceria-based ceramics.
Accordingly, it is a feature of the present invention to provide a miniaturized solid-oxide fuel cell and process for forming the fuel cell. This, and other features and advantages of the present invention, will become apparent from the following detailed description, the accompanying drawings, and the appended claims.